Structure and Function of P19, a High-Affinity Iron Transporter of the Human Pathogen Campylobacter jejuni

Abstract

Campylobacter jejuni, a major cause of acute bacterial diarrhea in humans, expresses numerous proteins to import diverse forms of essential iron. The expression of p19 and an adjacent iron transporter homologue ( ftr1) is strongly induced upon iron limitation, suggesting a function in iron acquisition. Here, we show that the loss of P19 alone is detrimental to growth on iron-restricted media. Furthermore, metal binding analysis demonstrates that recombinant P19 has distinct copper and iron binding sites. Crystal structures of P19 have been solved to 1.41 Å resolution, revealing an immunoglobulin-like fold. A P19 homodimer in which both monomers contribute ligands to two equivalent copper sites located adjacent to methionine-rich patches is observed. Copper coordination occurs via three histidine residues (His42, His95, and His132) and Met88. A solvent channel lined with conserved acidic residues leads to the copper site. Soaking crystals with a solution of manganese as iron analog reveals a second metal binding site in this solvent channel (metal–metal distance, 7.7 Å). Glu44 lies between the metal sites and displays multiple conformations in the crystal structures, suggesting a role in regulating metal–metal interaction. Dimerization is shown to be metal dependent in vitro and is detected in vivo by cross-linking.